FIG. 6 is a block diagram which schematically illustrates by way of example the structure of a conventional apparatus for detecting abnormality in a speed changing system.
As shown in the drawing, an output from an engine 41 is transmitted to a transmission 43 via a torque converter 42, and an output from the transmission 43 is then transmitted to driving wheels 44 via a differential gear unit and a speed reducing unit at a final stage both of which are not shown in the drawing.
The engine 41 is equipped with an engine rotation sensor 45 from which a signal is inputted into a controller 46 corresponding to the present rotational speed of the engine 41. In addition, each of the driving wheels 44 is equipped with a vehicle speed sensor 47 from which a signal is also inputted into the controller 46 corresponding to the present vehicle speed.
Subsequently, the controller 46 outputs a speed stage command to obtain a speed stage in conformity with the speed stage command so as to actuate a transmission clutch shift valve 48, causing a certain speed changing clutch in the transmission 43 to be selected and then brought in an engaged state.
Conventionally, the speed changing system is provided with an abnormality detecting apparatus for detecting abnormality of each of the engine rotation sensor 45 and the vehicle speed sensor 47 in response to outputs from these sensors 45 and 47.
This abnormality detecting apparatus is intended to detect the abnormality based on the result derived from comparison of an output from the engine rotation sensor 45 with an output from the vehicle speed sensor 47. For example, in a case where the present speed stage remains at a certain speed stage other than a neutral stage, the abnormality detecting apparatus determines whether an output from the engine rotation sensor 45 exceeds a threshold of 2500 r.p.m. or not, and moreover, determines whether an output from the vehicle speed sensor 47 is reduced to a level of zero kilometer or not. At this time, as shown in FIG. 7, when it is found that the output from the engine rotation sensor 45 exceeds the threshold of 2500 r.p.m. and the output from the vehicle speed sensor 47 is reduced to a level of zero kilometer, the abnormality detecting apparatus determines that "abnormality is present with respect to the vehicle speed sensor 47" (which represents that a vehicle speed signal system is abnormal).
Since the conventional abnormality detecting apparatus is constructed in the above-described manner, it makes the aforementioned determination on the assumption that the transmission clutch shift valve 48 is properly actuated and the speed changing clutch is properly brought in an engaged state.
When it is confirmed that the transmission clutch shifting valve 48 is properly actuated and the speed changing clutch is properly brought in an engaged state, it can be concluded that the aforementioned determination has been made correctly. However, in a case where the vehicle speed sensor 47 operates correctly but abnormality arises in a hydraulic system, there may be caused an occasion that an output from the engine rotation sensor 45 exceeds 2500 r.p.m. and an output from the vehicle speed sensor 47 is reduced to a level of zero kilometer even when the transmission clutch shift valve 48 fails to operate properly and the present speed changing stage in the transmission 43 remains still at a neutral stage. Also in this case, the abnormality detecting apparatus determines that "abnormality is present with respect to the vehicle sensor 47". This means that the abnormality detecting apparatus fails to exactly detects the practical position where the abnormality has arisen.
As is apparent from the above description, the conventional abnormality detecting apparatus has a problem that it is short of reliability, since it can not specifically identify the practical position where the abnormality has arisen.
The present invention has been made in consideration of the foregoing background and its object resides in providing an apparatus for detecting abnormality in a speed changing system wherein the apparatus assures that the position where abnormality arises in the speed changing system can specifically be identified with excellent accuracy.